1. Field of the Invention
The invention relates generally to radar and in particular to MTI (moving target indicator) radar.
2. Description of the Prior Art
Radars of the moving target indicator (MTI) type detect targets that are moving while suppressing radar returns from fixed targets. MTI radars are particularly useful in detecting a moving target in the presence of clutter, i.e. returns from uninteresting stationary targets which may overwhelm the return from the moving target of interest. Some MTI have the further capability of determining the velocity of the target.
Because the target is moving at a velocity v.sub.r relative to the radar, the frequency of the moving target return is shifted by the doppler frequency, f.sub.d =2.multidot.v.sub.r /.lambda., where .lambda. is the RF wavelength of the transmitted signal. Then, if the received radar signal is phase detected relative to a coherent intermediate frequency (IF) oscillator in the radar set, the detected phase varies between successive pulses because the position of the moving target has moved a corresponding phase length between pulses. Most MTI radars use a pulse-to-pulse comparison to extract moving targets from a stationary background. However, this procedure introduces several problems. First, multiple blind speeds are introduced. If the relative speed of the target is a multiple of PRF.multidot..lambda./2, where PRF is the pulse repetition frequency, then no phase difference, within factors of 2.pi., exist from pulse to pulse, and the target appears stationary although the signal is doppler shifted. Secondly, long interpulse periods, i.e. low PRFs, that are necessary to prevent range ambiguities permit distributed clutter echoes such as those from wind driven rain or chaff to decorrelate between pulses so that such echoes would not cancel upon comparison of two pulses. Thirdly, the repetitive pulsing requires long pulse trains at relatively low and fixed frequency, rendering difficult the use of high frequencies and frequency-agile radars.